There exist, known in the art, systems for polishing optical lenses. Some of these systems utilize a magnetorheological polishing substance known as a polishing slurry. Typically, the slurry is a mixture of magnetorheological compound, abrasive particles, and stabilizers.
When not acted upon by magnetic forces, the slurry is commonly in a liquid state. However, once acted upon by such a force, the slurry becomes much more viscous, pushing the abrasive particles to the surface of the liquid. This more viscous slurry, with the abrasives protruding from the surface, is then utilized as a polishing tool for abrading and polishing a work piece surface. Kardonsky et al. in U.S. Pat. Nos. 5,577,948 and 5,449,313 describes such a system.
In order for the magnetorheological-polishing device to be most efficient when being used as a polishing tool, it should be hard enough to apply sufficient force to press firmly the abrasive particles to the surface of the work piece. The polishing devices utilized in prior art systems acquire viscous, almost plasticized, properties known as Bingham properties, under the influence of magnetic forces. At this point, the device is hard enough to be used as a polishing tool. However, prior art devices reach this fully developed Bingham state only once, at the onset of the polishing movement.
Thereafter, once the movement of the work piece relative to the slurry commences, the slurry no longer sustains the Bingham properties, and the slurry loses the plasticized properties. Hence, although they become more viscous, they ultimately remain in the liquid state. Thus, frequently the liquid does not have sufficient force to push the submerged abrasive particles firmly against the polishing surface and, consequently, the abrasives do not efficiently abrade the work piece.
Additionally, polishing of the work piece is carried out in stages. At any given time a small surface area is polished. This area is defined by the size of a zone, known as the polishing zone, which is small relative to the size of the work piece. Thus the work piece is polished zone by zone. This approach hinders the achievement of uniform polishing across the entire surface of the work piece. Non-uniform work pieces, such as silicon wafers, present a potential problem in devices such as semi-conductors.
An additional problem is the non-uniformity of the magnetic field, which affects the affectivity of the polishing zone. The magnetic field on the magnet""s edge is almost an order of magnitude higher than that at the center of the magnet. Therefore, the visco-plastic properties of the slurry in the polishing zone are non-uniform, contributing to non-uniform polishing of the surface.